Nitric acid is an important commodity in the chemical industry and serves, for example, as the basis for production of fertilizers, explosives, and for nitration of organic substances in the production of dyes and disinfectants.
Since the early 20th century, nitric acid has been produced by the Ostwald process, on which large-scale industrial production has been based to date. This reaction is a catalytic reaction of ammonia. The nitrogen monoxide formed reacts to give nitrogen dioxide, from which reaction with water forms nitric acid which can be removed in trickle towers. This process is described in the publication “Anorganische Stickstoffverbindungen” [Inorganic Nitrogen Compounds] by Mundo/Weber, Carl Hanser Verlag Munich Vienna 1982, and in WO 01/68520 A1.
For preparation of nitric acid, ammonia NH3 is generally first reacted with air to obtain nitrogen oxide NO, which is then oxidized up to nitrogen dioxide NO2.
Subsequently, the nitrogen dioxide NO2 thus obtained is absorbed in water to form nitric acid. In order that a maximum amount of the nitrogen dioxide NO2 obtained is absorbed by water, the absorption is effected generally at elevated pressure, preferably at pressures between 4 and 14 bar.
The oxygen required for the conversion of the ammonia used as the raw material is generally supplied in the form of atmospheric oxygen. For the purpose of supply, the process air is compressed in a compressor and brought to a pressure appropriate both for the oxidation reaction and the absorption reaction.
Typically, the energy for compression of the air is obtained firstly by means of decompression of the residual gas leaving the absorption to ambient pressure in a residual gas expander, and secondly through the utilization, of the heat released in the reactions. The nitric acid plants constructed in various designs are matched to the specific requirements for the site of each one.
The preparation of nitric acid can be effected in a single pressure process or in a dual pressure process. In the single pressure process, both the combustion and the absorption are conducted at moderate pressure (<8 bar) or high pressure (>8 bar).
Single pressure processes are used especially when the required daily production is low. In these cases, the nitric acid plant is preferably operated by the mono high pressure process or by the mono medium pressure process. In the mono high pressure process, the combustion of the ammonia and the absorption of the nitrogen oxides are effected at about the same pressure of >8 bar. The advantage of the mono high pressure process is that a compact design is ensured.
In the mono medium pressure process, the combustion of the ammonia and the absorption of the nitrogen oxides are effected at about the same pressure of <8 bar. The advantage of the mono medium pressure process is that an optimal combustion yield is ensured.
If, in contrast, high nominal capacities and/or relatively high acid concentrations are required, a nitric acid plant executed by the dual pressure process is the more economical solution. In the dual pressure process, the combustion of the ammonia used is accomplished at a first pressure, namely at a lower pressure compared to the absorption pressure. The nitrous gases formed in the combustion are generally brought to the second pressure, the absorption pressure, after cooling by means of nitrous gas compression. The advantage of the dual pressure process is that the pressure stages are appropriate for the respective reactions and thus both an optimal combustion yield and a compact absorption are ensured.
In general, the plants for performance of the processes discussed above comprise at least one air compressor and at least one expansion turbine for the residual gas (also called “residual gas turbine”).
Such plants are known, for example, from WO 2009/146758 A1 and WO 2011/054928 A1.
In contrast to steady-state operation, in the startup and shutdown operation of nitric acid plants, the units present do not work under standard conditions and frequently require additional regulation.
In the course of startup from the switched-off/cold state, the nitric acid plant is generally first filled with air with the import of outside energy (for example outside steam or power) (“air operation”). The first emissions of NOx arise as soon as the absorption tower is filled with nitric acid from a reservoir vessel during the startup operation and the nitrogen oxides present in the acid are blown out by the air, and in modern plants the NOx formed during the filling operation is emitted. With the ending of the filling operation, the NOx emission then also ceases at first, until the NH3 oxidation in the nitric acid plant is started (“ignited”). After the ignition, the temperature and NOx concentration in the plant rise constantly to the steady-state operation value, and the individual plant parts can be operated as planned from a particular time.
In the shutdown of the nitric acid plant, the NH3 oxidation is first stopped. The NOx concentration at the outlet from the absorption tower decreases constantly and the temperature falls in parallel thereto. Here too, from a certain time, individual plant parts can no longer be operated as planned since the steady-state operation values can no longer be complied with.
It is known from Dutch Notes on BAT for the Production of Nitric Acid, Final Report, Ministry of Housing, Spatial Planning and the Environment: The Hague, NL, 1999 that NOx emissions during startup and shutdown can be reduced by heating the residual gas. In addition, it is suggested to the person skilled in the art that this can be achieved by means of a steam heater.
In the operation of the nitric acid plant, it is desirable to achieve a high efficiency of the residual gas turbine in order to lower the operating costs. For this purpose, the inlet temperature of the medium which flows through the residual gas turbine during the startup or shutdown operation of the plant must be sufficiently high that the gases leaving the residual gas turbine do not freeze. Especially in the case of residual gas turbines with high efficiency, there is an increased risk of freezing, since a residual gas turbine with improved efficiency cools the medium which flows through it much more significantly for the same inlet temperature compared to a conventional residual gas turbine.
In contrast to normal operation of the plant, the inlet temperature of the medium flowing through the residual gas turbine is usually lower during startup and/or shutdown.
Therefore, the efficiency of the residual gas turbine is limited by the medium flowing through it in the course of startup and/or shutdown in order to prevent the freezing of the residual gas turbine during startup and/or shutdown.
Especially in the case of plants comprising residual gas turbines with particularly high efficiency, it is necessary, at least during the startup and shutdown phases of the plant, to heat the medium which is present on the residual gas side and is fed into the residual gas turbine.